Regex: move boost headers to top of list to work around clang issue which requires workarounds present in config.hpp.

test/c_compiler_checks/wide_posix_api_check.c

@@ -20,11 +20,11 @@
 #define UNICODE
 #define _UNICODE
 
+#include <boost/regex.h>
 #include <stdio.h>
 #include <string.h>
 #include <assert.h>
 #include <stdlib.h>
-#include <boost/regex.h>
 
 #ifndef BOOST_NO_WREGEX
 #include <wchar.h>